Biotic Influences: Parasitic Associations

Abstract

We have so far mainly dealt with autotrophic plants that assimilate CO₂ from the atmosphere into complex organic molecules and acquire nutrients and water from the rhizosphere. There are also fascinating plant species that lack the capacity to assimilate sufficient CO₂ to sustain their growth and that cannot absorb nutrients and water from the rhizosphere in sufficient quantities to reproduce successfully. These plants comprise approximately 1% of all flowering plant species; they are parasitic and rely on a host plant to provide them with the materials they cannot acquire from their abiotic environment (Nickrent et al. 1998; Westwood et al. 2010). About 4000 plant species within 270 genera in over 20 families [predominantly angiosperms; we only have firm evidence for one gymnosperm parasite: Parasitaxus ustus (conifer coral tree) (Feild and Brodribb 2005)] rely on a parasitic association with a host plant for their mineral nutrition, water uptake, and/or carbon supply (Fig. 15.1). Molecular analysis of nuclear, chloroplast, and mitochondrial DNA suggests at least 12 or 13 origins of parasitism in angiosperms (Nickrent et al. 1998; Barkman et al. 2007; Westwood et al. 2010). They inhabit ecosystems ranging from the high Arctic to the tropics (Press and Phoenix 2005). Some of these species [e.g., Striga spp. (witchweed), Orobanche spp. (broomrape), Cuscuta spp. (dodder laurel), and Arceuthobium douglasii (Douglas-fir dwarf mistletoe)] are economically important pests that cause large yield losses of crop or forest plants, especially in Africa and Mediterranean countries (Estabrook and Yoder 1998; Runyon et al. 2010). Other parasitic species (Cistanche spp.) are grown commercially to extract traditional medicines in China (Li et al. 2016), or for their fragrant wood [Santalum album and Santalum spicatum (sandalwood)] (Jones et al. 1995). Ecologically, parasitic plants fill a fascinating niche in their exploitation of other plants to acquire sparingly available resources.